The invention relates to a method for operating a current converter, in particular an electric machine, with which, for the or for each semiconductor switch of the current converter, a control signal for setting a switch-off speed of the semiconductor switch is generated. The invention also relates to a current converter operating in accordance with this method, and to an electric machine comprising a current converter of this kind.
Motor vehicles driven electrically (by electric motor), for example electric or hybrid vehicles, typically have electric or electromotive machines for driving one or both motor vehicle axles. Electromotive drive machines of this kind generally comprise a controlled synchronous or asynchronous motor as electric motor, which is coupled to an energy store within the vehicle (high-voltage battery) in order to be supplied with electrical energy.
The electric motor of the electric machine usually comprises a rotor mounted rotatably relative to a stator, which rotor is driven by means of a magnetic rotary field. In order to generate the rotary field, the coils (phase windings, stator windings) of the stator are acted on by an appropriate three-phase current (alternating voltage) as motor current, which is converted from a direct voltage or a direct current (input current) of the energy store by means of a current converter.
Current converters of this kind on the one hand are suitable and designed for converting, as inverters, the input current of the energy store into the motor current in normal operation of the machine. On the other hand, the current converters are generally suitable and designed for converting, as rectifiers, a generated motor current of the electric motor into a direct current (regenerative current) for feeding into the energy store in generative or recuperative operation.
For this purpose, the current converter has a bridge circuit (intermediate circuit, commutation circuit) coupled to an intermediate circuit capacitor. The bridge circuit comprises a number of bridge modules (half-bridges, power module, commutation cell) corresponding to the (motor) phases, said bridge modules comprising semiconductor switches which are connected between an outward line and a return line of the current converter.
In order to control the semiconductor switches of the bridge modules, driver circuits are generally provided, which generate pulse width modulated control signals (PWM signals) depending on signals of a controller (motor control unit). The control signals are guided to a control connection of the semiconductor switching question, which is thus switched in a clocked manner between a conductive (connected) state and a non-conductive (disconnected, blocking) state. In operation the semiconductor switches, embodied for example as IGBTs (insulated gate bipolar transistors), are switched in a clocked manner via their respective gate connections to the control signals, whereby the collector or load current to be switched is consequently converted into the three-phase current for the drive of the rotor.
The electrical power loss of a current converter of this kind is determined substantially by conduction and switching losses if the bridge modules or semiconductor switches thereof. Here, the (gate) control method characterised by the control signals has a significant influence on the magnitude of the power losses that occur—and therefore on the efficiency—of the current converter. The semiconductor switches generally have a comparatively slow switch-off speed, which signifies a long (switch-off) duration of a switch-off process from the conductive state into the non-conductive state. In operation, this results in an undesirably high proportion of electrical losses during the switch(over) processes of the semiconductor switches.
in a switch-off process of this kind the collector or load current to be switched, which flows through the semiconductor switch in the conductive state, is reduced until it is switched off in essence. The switch-off speed is defined here substantially by the gradient of the resultant current flank of the load current, by which the duration of the switch-off processes determined. This switch-off speed can be set or controlled by open-loop and/or closed-loop control by means of the control signal for (gate) control of the semiconductor switches.
Document DE 10 2014 219 470 A1 discloses a method for the temperature-dependent selection of switch-off speeds for switch-off processes in semiconductor switches. There, the semiconductor switches are arranged in a DC intermediate circuit of a converter. In operation a direct voltage of the DC intermediate circuit is measured and compared to a reference direct voltage. On the other hand, a temperature, in particular a barrier layer temperature, of the semiconductor switches is measured and compared to a reference temperature. A switch-off speed for the semiconductor switches is set on the basis of the comparisons. Quicker switch-off speeds of the semiconductor switches should thus be made possible for specific working or operating points of the converter.
The object of the invention is to describe a particularly suitable method for operating a current converter. A further object of the invention is to describe a current converter operating by a method of this kind, and an electric machine comprising a current converter of this kind.